Postformable laminate comprising a barrier sheet of wood fiber impregnated with fully cured unplasticized melamine-formaldehyde resin

ABSTRACT

A laminate which is postformable to an arc of a circle having a radius at least as small as three-fourths inch without cracking or dulling of the surface is produced with a paper barrier sheet of wood fiber impregnated with a fully cured unplasticized malamine-formaldehyde resin by consolidating under heat and pressure a postformable substrate in direct contact with a paper barrier sheet consisting essentially of alpha cellulose impregnated with an unplasticized reaction product of formaldehyde and malamine in a mole ratio from 1.1:1 to 1.8:1. The reaction product has an acidity high enough to compensate for its low formaldehyde content, so as to cause it to become fully cured during the heat-and pressure-consolidation.

Unite States Patent 1191 Meiser 1451 May 1,1973

[75 Inventor: Kenneth D. Meiser, Dallas, Tex.

[73] Assignee: Plastics Manufacturing Company,

Dallas, Tex.

[22] Filed: Mar. 23, 1971 [21] Appl. N01: 127,412

Related US. Application Data [63] Continuation-impart of Ser. Nos.535,733, March 21, 1966, abandoned, and Ser. No. 9,456, Feb. 6, 1970,abandoned.

[51] Int. Cl. ..B32b 27/10 [58] Field of Search ..161/257, 258, 259,161/263, 264, 261, 413; 156/60, 196, 212,

[56] References Cited UNITED STATES PATENTS 3,294,622 12/1966 Wark161/264 2,773,788 12/1956 Magrane et a1 ..161/263 X 2,331,446 10/1943Widmer et a1 .260/69 2,841,571 7/1958 Wohnsiedler ...260/67.6 3,372,0843/1968 Arledter 162/1 28 3,318,760 5/1967 Boenig ..161/263 3,131,1164/1964 Pounds ..161/258 Primary Examiner-Daniel ,1. F ritschAttorneyMarsha1l & Yeasting 5 7 ABSTRACT A laminate which ispostformable to an arc of a circle having a radius at least as small asthree-fourths inch without cracking or dulling of the surface isproduced with a paper barrier sheet of wood fiber impregnated with afully cured unplasticized malamine-formaldehyde resin by consolidatingunder heat and pressure a postformable substrate in direct contact witha paper barrier sheet consisting essentially of alpha celluloseimpregnated with an unplasticized reaction product of formaldehyde andmalamine in a mole ratio from 1.111 to 1.8:]. The reaction product hasan acidity high enough to compensate for its low formaldehyde content,so as to cause it to become fully cured during the heat-andpressure-consolidation.

5 Claims, No Drawings CROSS-REFERENCE TO RELATED APPLICATIONS This is acontinuation-in-part of application Ser. No.

535,733, filed Mar. 21, 1966 and application Ser. No.

9,456, filed Feb. 6, 1970, both of which are now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an improved postformable laminate, produced by consolidatingunder heat and pressure a postformable substrate in direct contact witha decorative barrier sheet providing an attractive outer surface.

In the production of such a postformable laminate, the criticalproblemis the problem of providing a barrier sheet that will not crackwhen an attempt is made to postform the laminate.

In the production of flat decorative laminates, for many years thedecorative surface sheet has consisted of a sheet of paper impregnatedwith a melamine-formaldehyde resin, which provides a surface ofexcellent appearance and durability.

However, it is well known, as pointed out in the prior art patents, thata melamine-formaldehyde resin impregnated paper sheet of the ordinarytype which is used as the barrier sheet in flat laminates cannot be usedas the barrier sheet in a postformable laminate, because such a barriersheet would crack during the postforming of the laminate.

2. Description of the Prior Art Each of the prior patents, like thepresent invention, is directed to the problem of providing a decorativebarrier sheet for a postformable laminate which will not crack when anattempt is made to postform the laminate. 7

Each of the prior patents explains that an ordinarymelamine-formaldehyde impregnated paper sheet, when used as the barriersheet for a postformable laminate, will crack when an attempt is made topost form the laminate, and each of the prior patents discloses someexpedient for preventing the decorative barrier sheet from crackingduring the postforming of the laminate.

The expedients for preventing the barrier sheet in a postformablelaminate from cracking during postforming which are disclosed in theprior patents are as follows:

a. use of expensive long fibers in the barrier sheet; (H. Arledter U.S.Pat. No. 3,372,084)

b. incomplete curing of melamine-formaldehyde resin in barrier sheet;(Pounds U.S. Pat. No. 3,131,116)

0. addition of water as a plasticizer for the melamineformaldehyde resinin the barrier sheet; (Wark U.S. Pat. No. 3,294,622)

d. modification of the melamine-formaldehyde resin in the barrier sheetby incorporation of a chemical plasticizer; (Magrane U.S. Pat. No.2,773,788).

Each of these four expedients tends to minimize cracking of amelamine-formaldehyde impregnated barrier sheet during the postformingof a laminate. However, each of these four expedients has certainserious disadvantages as explained below.

Heretofore it was believed that in order to prevent amelamine-formaldehyde impregnated barrier sheet from cracking during thepostforming of a laminate, it

0 was necessary either (a) to use a paper barrier sheet containing asubstantial proportion of long fibers, or to use a melamine-formaldehyderesin which was either (b) incompletely cured, or modified by theincorporation of (0) free water as a plasticizer, or (d) a chemicalplasticizing agent.

The principal object of the invention is the production of apostformable laminate having a paper barrier sheet of ordinary woodfiber impregnated with a fully cured unplasticized melamine-formaldehyderesin,

which is postformable to an arc of a circle having a.

radius at least as small as three-fourths inch without cracking ordulling of the surface.

a. Disadvantages of Using Long Fibers in the Barrier Sheet H. ArledterU.S. Pat. No. 3,372,084 states in lines 59-63 of column 8, All of thefibers yielding good post-formability in papers according to thisinvention are more expensive than regular cellulosic wood pulp such asalpha, sulphite, or sulfate.

One object of the present invention is to use in a postformable laminatea paper barrier sheet consisting essentially of the inexpensive regularcellulose wood pulp.

H. Arledter U.S. Pat. No. 3,372,084 also states, in lines 58-63 ofcolumn 1, Heretofore post-formability of laminates was limited tolaminate thicknesses of 0.045 to 0.050 inch, and bend radii offive-eighths inch or higher, and to achieve this limited goal, specialpostformable melamine-formaldehyde resins and specific cross-crepedcorestock inlays were required. These special post-fonnablemelamine-formaldehyde resins" are the incompletely cured resins and theplasticized resins.

b. Disadvantages of Using an Incompletely Cured Melamine-formaldehydeResin in the Barrier Sheet Pounds U.S. Pat. No. 3,131,116 discloses apostformable laminate having an incompletely cured melamine-formaldehyderesin in the barrier sheet. It is well known that an incompletely curedmelaminedormaldchyde resin is inferior in water-resistance anddurability to a completely cured melamine-formaldehyde I'CSIII.

In the postformable laminate of the present invention, themelamineformaldehyde resin with which the barrier sheet is impregnatedis fully cured.

c. Disadvantages of Plasticizing the MelamineFormaldehyde Resin in theBarrier Sheet By Addition of Water Wark U.S. Pat. No. 3,294,622discloses a postformable laminate in which the melamine-formaldehyderesin impregnated barrier'sheet contains an excess of free water, as aplasticizing agent. The excess of free water acts as a plasticizingagent to prevent the barrier sheet from cracking during postforming, butalso partially hydrolyzes and degrades the melamine-formaldehyde resinin the barrier sheet. Also, the amount of free water in the barriersheet would be difficult to control because of the loss of free water byevaporation.

Wark U.S. Pat. No. 3,294,622 states, in lines 9, l and 70 of column 4,that the added moisture migrates during the pressing into theimpregnated sheets, and that this water imparts improved flow propertiesto the resins. The December 1946 issue ofPlastics, on page 95 states:

Because melamine resin treated materials tend to be brittle it may alsobe necessary, in certain instances, to plasticize the resin treatedmaterials, either by the use of steam or high humidity, or the use ofcertain chemicals in the resin varnish.

The action of added water in plasticizing melamineformaldehyde resins iswell known, and it is also well known that the use of added water as aplasticizer is disadvantageous in that it partially hydrolyzes anddegrades a melamine-formaldehyde resin.

d. Modification of the Melamine-Formaldehyde Resin in the Barrier SheetBy Incorporation of A Chemical Plasticizing Agent The expedient whichhas been used conventionally in order to prevent the cracking duringpostforming of a melamine-formaldehyde resin impregnated barrier sheetin a postformable laminate has consisted in modifying themelamine-formaldehyde resin in the barrier sheet by incorporation of achemical plasticizing agent.

A chemical plasticizing agent which has commonly been incorporated inorder to plasticize a melamineformaldehyde resin with which the barriersheet of a postformable laminate is impregnated is toluene sulfonamide,which is reacted with melamine and formaldehyde in order to produce aplasticized or modified melamine-formaldehyde resin as described inMagrane U.S. Pat. No. 2,773,788.

The use of other chemical plasticizing agents to modify themelamine-formaldehyde resin with which the barrier sheet of apostformable laminate is impregnated is disclosed in U.S. Pat. Nos.2,584,177, 2,937,966, 3,067,077, 3,082,180 and 3,194,723. Pounds U.S.Pat. No. 3,131,116 states, in lines 63-73 of column 3:

Another method practiced by the prior art in preparing post-formablelaminates consists of more or less extensively modifying theformaldehyde condensates of melamine used to impregnate the print andoverlay members of the assembly. The disadvantage residing in thismethod is that when an effective amount of the modifier or plasticizeris used in conjunction with the melamine resin, whether the plasticizingis internal or otherwise, there results an inferior degree of heat andchemical resistance associated with the cured products derived from suchcompositions."

Because of the degrading action of the chemical plasticizing agents withwhich the melamine-formaldehyde resin in the barrier sheet ofpostformable laminates has been modified, it is customary to limit theamount of the plasticizing agent so that the resulting laminate is notfully postformable in that the radius of each bend must be keptrelatively large in order to avoid cracking of the surface lamination.In this way, a compromise has been struck between poor postformabilityof the laminate and poor durability of the surface.

In order to show the poor postformability of a conventional postformablelaminate in which the barrier sheet is impregnated with a conventionaltoluene sulfonamide-melamine-formaldehyde resin, such a conventionalpostformable laminate is used as the control in the examples hereinaftersetforth.

SUMMARY OF THE INVENTION The present invention is based upon thesurprising discovery that, contrary to the teaching of the prior art, itis possible to produce a laminate having excellent postformability inwhich the paper barrier sheet (a) consists essentially of ordinary alphacellulose wood fiber impregnated with a (b) fully cured, (c)unplasticized melamine-formaldehyde resin, provided that the mole ratioof formaldehyde to melamine used in the melamine-formaldehyde resin iswithin a certain narrow critical range, and provided that the acidity ofthe melamine-formaldehyde resin during consolidation under heat andpressure is high enough to compensate for its low formaldehyde contentso as to cause it to become fully cured. A laminate produced inaccordance with the present invention also has excellent waterresistance, chemical resistance and heat re sistance.

Critical Range of Formaldehyde-Melamine Ratios In the practice of thepresent invention, in order to produce a postformable laminate in whichthe paper barrier sheet consists essentially of ordinary alpha cellulosewood fiber impregnated with a fully cured unplasticizedmelamine-formaldehyde resin, it is necessary that the formaldehyde andmelamine be reacted in a mole ratio from 1.1:1 to 1.811. This narrowcritical range is in contrast to the wide ranges of 1:1 to 20:1disclosed by Widmer U.S. Pat. No. 2,331,446, and 1:1 to 6:1 disclosed byMagrane U.S. Pat. No. 2,773,788. In order to provide a reasonably rapidrate of cure, it is customary to use a molar formaldehyde-melamine ratioof at least 2:1, as described in all of the specific examples of theWidmer and Magrane patents.

The melamine-formaldehyde resin with which the barrier sheet isimpregnated in the practice of the invention is very slow curing becauseof its low formaldehyde content, and would not become fully cured ifsubjected to ordinary curing conditionsv during the heatandpressure-consolidation of the laminate. In the practice of theinvention, however, the curing of the melamine-formaldehyde resin withwhich the barrier sheet is impregnated takes place at an increasedacidity, which is high enough to compensate for the low formaldehydecontent of the resin and to cause the resin to become fully cured duringthe heatand pressureconsolidation of the laminate.

It has been discovered that when the resin with which the barrier sheetis impregnated is a reaction product of formaldehyde and melamine in amole ratio from 1.1:] to 1.811, and has an acidity high enough tocompensate for its low formaldehyde content so as to cause the reactionproduct to be completely cured during the heatand pressure-consolidationof the laminate, the resulting laminate is unique in that it can bepostformed without cracking of the barrier sheet even though the barriersheet consists essentially of ordinary alpha cellulose wood fiberimpregnated with an unplasticized, fully cured melamine'formaldehyderesin. This is a surprising result, because it has been generallyaccepted for many years, as stated in the prior patents that a laminatein which the barrier sheet is an alpha cellulose sheet impregnated withan unplasticized, fully cured melamine-formaldehyde resin will alwayscrack when an attempt is made to postform it. i

The non-brittleness of the resin with which the barrier sheet has beenimpregnated in the present laminate is surprising, because the use ofhigher acidity to compensate for the low formaldehyde content of theresin and to force the resin to cure completely during heatandpressure-consolidation might have been expected to produce a morebrittle resin, whereas the resin in the barrier sheet of the presentlaminate is actually less brittle in postforming than any unplasticizedfully cured melamine-formaldehyde resin heretofore used in the barriersheet of a postform able laminate.

Control of Acidity in Order to Compensate for Low Formaldehyde Contentof Formaldehyde-Melamine Resin in Barrier Sheet In the practice of thepresent invention, the barrier sheet which is consolidated with apostformable substrate is impregnated with an unplasticized reactionproduct of formaldehyde and melamine in a mole ratio from 1.1:1 to1.8:1, having an acidity high enough to compensate for its lowformaldehyde content so as to cause it to become fully cured duringheatand pressure-consolidation of the laminate.

In order to determine the proper acidity which is required in order tocause the formaldehyde-melamine reaction product to become fully curedin a particular case, several samples of a laminate are prepared byheatand pressure-consolidation, the samples being identical except thatthe acidity of the formaldehydemelamine reaction product with which thebarrier sheet is impregnated is varied. The samples of laminate thusproduced are then tested in order to determine what degree of acidity ofthe formaldehyde-melamine reaction product is required in order toproduce a laminate that passes the cure test. The cure test employed isa test of the water resistance of the laminate in accordance with theprocedure described in NEMA Standards Publication No. LD-l964, entitledLaminated Thermosetting Decorative Sheets," section LP2-2.l I. In thistest, a standard metal cup filled with water is heated until the waterboils, a small amount of the boiling water is then poured onto thesurface of the laminate, and the cup containing the remainder of the hotwater is then immediately placed on the wet surface. After minutes, thelaminate is wiped dry and the portion of the laminate which was coveredby the cup during the 20-minute period is examined to determine whetheror not it has been affected by the water. If no deterioration of thesurface is observed, the resin in the barrier sheet is considered to befully cured.

The above test, which determines whether or not the acidity of theformaldehyde-melamine reaction product with which the barrier sheet isimpregnated is high enough to cause the reaction product to become fullycuredduring the consolidation of the laminate, is used instead ofattempting to measure the absolute degree of acidity of the reactionproduct, because the acidity itself may be affected by various factorssuch as the acidity of the paper which is impregnated with the reactionproduct to form the barrier sheet. Also, the pH required to cause thereaction product to cure completely under given conditions oftemperature and pressure may vary, depending upon the nature of thesubstance or substances used to furnish the acidity.

Control of the acidity to cause the complete cure of theformaldehyde-melamine reaction product with which the barrier sheet isimpregnated is illustrated by the preparation of the followinglaminates:

Laminate A A reaction flask equipped with a thermometer, stirrer andreflux condenser was charged with 126 parts of melamine, 121.5 parts ofa 37 percent aqueous solution of formaldehyde, 53 parts of water andenough sodium hydroxide solution to bring the pH of the resultingslurrybetween 7.8 and 8.2. The slurry was heated for 20 minutes to bring it toreflux temperature, and refluxing was then conducted for an additionalperiod of 30 minutes. The resinous solution was then cooled to roomtemperature. This solution, which contained an unmodified reactionproduct of formaldehyde and melamine in a mole ratio of 1.521, was usedto saturate a sheet of pigmented alpha cellulose paper, and thesaturated sheet was then dried for seconds at 255 F. When a sample ofthe resulting sheet was tested by heating it in a circulating-air ovenat 300 F. for ten minutes, it was found to have incurred a weight lossof 5 percent of the weight of the dried impregnated sheet. Accordingly,the volatile content of the sheet was considered to be 5 percent. Bycomparison with the dry weight of the original sheet, the sample of theimpregnated sheet after elimination of volatiles was found to contain 52percent by weight of the formaldehydemelamine reaction product. In orderto determine the pH level of the formaldehyde-melamine reaction productin the impregnated sheet, a sample of the impregnated sheet was immersedin distilled water for several minutes, and the pH of the distilledwater, containing the redissolved reaction product, was found to be 7.0.

A laminate was then prepared by assembling a bottom layer of kraftpaper, a second layer of crepe kraft paper which had been impregnatedwith a conventional postformable phenol-formaldehyde resin, four sheetsof ordinary kraft paper impregnated with the same phenolformaldehyderesin, and a top sheet cut from the impregnated sheet prepared ashereinbefore described, containing an unmodified formaldehyde-melaminereaction product in a mole ratio of 1.5:1. The assembly of sheets wasplaced in a laminating press under a pressure of 1,000 pounds per squareinch. After being placed under pressure, the assembly was heated to atemperature between 260 and 265 F., the time required to reach 260 F.being 20 minutes. After the assembly had been held under pressure at atemperature between 260 and 265 F. for 15 minutes, the assembly wascooled to about F. while still under a pressure of 1,000 pounds persquare inch. Such cooling was accomplished by passing cold water throughjackets provided on the press, and the time required for such coolingwas about 20 minutes.

The laminate was then tested for water resistance as hereinbeforedescribed, in accordance with the procedure described in NEMA StandardsPublication solution No. LDl-l964, entitled Laminated ThermosettingDecorative Sheets, Section LP2-2.l l. The surface of the laminate wasunaffected by this test, indicating that the acidity of theformaldehyde-melamine reaction product with which the barrier sheet wasimpregnated was high enough to compensate for its low formaldehydecontent, so as to cause the reaction product to become fully curedduring the heatand pressure-consolidation of the laminate.

Laminate B The procedure used in producing this laminate was the same asthe procedure used in producing Laminate A, except that the amount ofthe 37 percent aqueous solution of formaldehyde was 137.7 parts, theamount of water added to the initial mixture was 29 parts, and theperiod of refluxing was minutes. The mole ratio of formaldehyde tomelamine was 1.711. With this mole ratio, the acidity of the reactionproduct in the impregnated barrier sheet should by such that when asample of the impregnated sheet is soaked in distilled water it impartsto the distilled water a pH of about 7.5, instead of the pH of 7.0 whichwas shown by the same test in the preparation of Laminate A. The loweracidity in the present case is desirable because theformaldehyde-melamine reaction product in a mole ratio of 1.7:1 curesmore rapidly because of its higher formaldehyde content than theformaldehyde-melamine reaction product in a mole ratio of 1.5:1 whichwas present in the barrier sheet of Laminate A. The higher pH can beobtained in the present case either by using a more alkaline or morehighly buffered cellulosic paper for impregnation with the reactionproduct to produce the barrier sheet, or by raising the pH of thereaction product slightly by addition of sodium hydroxide before thesolution of the reaction product is used to impregnate the paper. Theuse of this higher formaldehyde-melamine mole ratio is advantageous whenthe paper to be impregnated is an alkaline paper, because a preparedwith a lower formaldehydemelamine mole ratio, if used to impregnatealkaline paper, would require the addition to the solution of an acidicsubstance to compensate for the lower formaldehyde content and to permitcomplete cure, and such addition of an acidic substance would tend toreduce the storage stability of the aqueous solution.

Laminate C Laminate C is produced by a procedure which is the same asthat used for producing Laminate A, except that the amount of the 37percent aqueous formaldehyde solution is 105.3 parts, the amount ofwater added is 92 parts and the period of reflux is 45 minutes. Also,the solids content of the solution is increased in this case to about 60percent, by applying a vacuum of inches at the end of the period ofrefluxing and allowing the solution to boil without application of heatso as to remove about 65 parts of water. The mole ratio of formaldehydeto melamine is 1.3:]. A sample of the paper impregnated with thissolution, when soaked in distilled water, should impart to the distilledwater a pH between 4.0 and 5.5. This higher acidity is required becausethe present reaction product, because of its lower formaldehyde content,is slower-curing, and can be obtained by using an acidic paper, or byreducing the pH of the solution of the reaction product before it isused to impregnate the paper. Any desired acid or acidic substance, suchas lactic acid or p-toluene sulphonic acid, may be added to the solutionof the reaction product in order to increase its acidity.

The term parts wherever used herein, means parts by weight.

Preparation of Unplasticized Formaldehyde-Melamine Reaction Product Theterm unplasticized is used herein to indicate that theformaldehyde-melamine reaction product with which the barrier sheet isimpregnated is prepared without the addition of any substance that hasan appreciable plasticizing action upon the formaldehydemelaminereaction product. In prior methods of producing postformable laminates,it has been customary to add modifiers, including water and variousother substances, which plasticized the formaldehydemelamine reactionproduct, but which also react chemically with the formaldehyde-melaminereaction product and which invariably reduce the water resistance anddurability of the surface of the laminate.

In the practice of the present invention, the formaldehyde-melaminereaction product may be prepared by dissolving one mole of melamine inan aqueous solution containing from 1.1 to 1.8 moles of formaldehyde andthen refluxing until a solution of the desired viscosity is obtained.The pH of the slurry that is obtained upon addition of the melamineshould be about 8. When the mole ratio of formaldehyde to melamine usedfor the reaction is in the upper part of the range between 1.121 and1.8:l, the total quantity of water in the initial reaction mixture maybe slightly less than the weight of the melamine. However, when the moleratio of formaldehyde to melamine is in the lower part of this range,the quantity of water present in the initial mixture should besubstantially greater than theweight of the melamine, in order that aclear or substantially clear solution of the reaction product may beobtained.

As the mole ratio of formaldehyde to melamine used for the reactionapproaches 1.8, the benefits obtained in the practice of the inventiondecrease slightly. On the other hand, as the mole ratio approaches1.1:1, it becomes more difficult to obtain a clear impregnatingsolution. The preferred mole ratio of formaldehyde to melamine is fromabout 1.2:1 to about 1.7:1.

The concentration of an aqueous solution of a formaldehyde-melaminereaction product, prepared as hereinbefore described, may be anyconcentration that makes it possible to impart the desired amount of thereaction product by impregnating the paper which is to be used to formthe barrier sheet of a postformable laminate embodying the presentinvention. The concentration of the solution may vary from about 30percent to about percent solids.

If desired, the reaction product may be separated by spray drying, thethe spray dried reaction product may be redissolved subsequently inwater to prepare an impregnating solution of the desired concentration.

Postformable Substrate The advantages of a laminate embodying thepresent invention become evident only when the laminate is postformed,and postforming is possible only when the barrier sheet of paperimpregnated with an unplasticized reaction product of formaldehyde andmelamine in a mole ratio of 1.1:1 to 1.821 is combined with apostformable substrate. The postformable substrate may be prepared inthe known manner, for example from sheets of kraft paper impregnatedwith a postformable phenol-formaldehyde reaction product. The content ofthe postformable phenol-formaldehyde reaction product in such sheets ofimpregnated kraft paper usually ranges from about 25 percent to about 35percent of the weight of the impregnated sheets.

Barrier Sheet A barrier sheet to be used in a postformable laminateembodying the present invention is prepared by impregnating a sheet ofalpha cellulose paper with a solution of an unplasticized reactionproduct of formaldehyde and melamine in a mole ratio between l.l:l and1.8: l prepared as hereinbefore described.

If desired, the alpha cellulose paper employed may be a pigmented paper.

The paper to be used for the barrier sheet may be impregnated by meansof conventional apparatus. A continuous web of paper may be fed from asupply roll, and may be impregnated by means of coating rolls, or bypassing it beneath the surface of the impregnating solution, or by byboth means.

After the impregnating operation, the impregnated paper may be dried ata temperature from about 240 F. to about 280 F. for about one minute.

The purpose of the barrier sheet is to provide a decorative surface ofsatisfactory appearance and durability, which hides the unattractivecolor of the substrate. Thus the barrier sheet is usually pigmented toincrease its hiding power.

Production of Laminate Postformable laminates are widely used forapplications such as the manufacture of counter tops and table topshaving contoured edges. These laminates are manufactured originally inthe form of flat sheets which are sold to manufacturers of furniture andcabinets. In the manufacture of furniture and cabinets, these laminatesare cut to the desired sizes and are postformed wherever bends arerequired.

A postformable laminate embodying the present invention is prepared fromone or two top sheets, and a plurality of sheets which form thepostformable substrate and which preferably consist of kraft paperimpregnated with a postformable phenol-formaldehyde reaction product ashereinbefore described. The substrate imparts strength and rigidity tothe laminate, and the top sheet or sheets provide an attractive anddurable surface.

The barrier sheet which overlies the core sheets often is called a printsheet because it is customarily printed with a non-bleeding ink in adecorative design, or is pigmented or dyed to produce a uniform color.

The color of the phenol-formaldehyde resin used to impregnate the coresheets in the substrate is poor and is not light fast. Therefor thebarrier sheet should be opaque or pigmented to hide the color of thecore sheets, and should be impregnated with a sufficient amount of aformaldehyde-melamine reaction product to prevent thephenol-formaldehyde reaction product of the core sheets from bleedinginto the barrier sheet.

In order to provide a surface of maximum durability and to protect thebarrier sheet against abrasion, an overlay sheet" is commonly used. ontop of the barrier sheet in a laminate. When an overlay sheet is used ina postformable laminate embodying the present invention, the overlaysheet consists of alpha cellulose paper impregnated with anunplasticizedl, fully cured reaction product of formaldehyde andmelamine in a mole ratio from l.l:l to 1.821, prepared as hereinbeforedescribed. Preferably such reaction product constitutes from about toabout percent of the weight of the overlay sheet. The overlay sheet isunpigmented so that in the finished laminate the overlay sheet issubstantially transparent.

Regardless of whether or not an overlay sheet is used, the barrier sheetis a postformable laminate embodying the invention should consist ofalpha cellulose paper impregnated with an unplasticized, fully curedreaction product of formaldehyde and melamine in a mole ratio from 1.111to 1.8:1.

Preferably about 40 to 60 percent of the weight of the barrier sheetconsists of a formaldehyde-melamine reaction product. The paper which isimpregnated to form the barrier sheet preferably consists of apigmented, relatively pure, absorbent alpha cellulose paper.

In the production of a postformable laminate embodying the presentinvention, the assembly of sheets is placed in a laminating press and issubjected to a pressure and temperature of the order usually employed inthe production of a high pressure laminate. The usual pressure is fromabout 800 to about l,500 pounds per square inch.

While the assembled sheets are held under pressure in the laminatingpress, they are heated to a temperature between 260 and 270 F. and areheld in this temperature range for a curing time ranging from about 10to about 20 minutes. These conditions effect the complete cure of theformaldehyde-melamine reaction product present in the top sheet orsheets. As the final step, the laminate is cooled under pressure to atemperature of about F.

A typical postformable laminate embodying the present invention has athickness of about 50 to 60 mils. Of this thickness, the overlay sheetcontributes about 2 to 4 mils and the barrier sheet contributes about 5to 9 mils. The postformable substrate accounts for the remainder of thethickness of the laminate.

A postformable laminate embodying the present invention may becharacterized as a decorative" laminate because of the fact that thesurface lamination contains a reaction product of formaldehyde andmelamine, which makes it possible for the surface of the lamination tohave an attractive and light-fast color.

The operation of postforming a laminate is conducted by heating thelaminate along the line of the desired bend to a temperature of about325 and 350 F., bending the laminate to the desiredcontour, and thenholding the laminate in its bent form while it cools. In this manner itis possible to produce a table top having a contoured edge, or a sinktop having a vertical splash shield which forms an unbroken continuationof the sink top.

Postformability The principal characteristic that distinguishes alaminate embodying the present invention from the postformabledecorative laminates heretofore known is that a laminate of the presentinvention is capable of being postformed to a smaller radius than thepostformable decorative laminates heretofore known, without cracking ordulling of the barrier sheet.

At the same time, the barrier sheet in a laminate of the presentinvention contains an unplasticized, fully cured reaction product offormaldehyde and melamine. For this reason, the durability, waterresistance, chemical resistance and heat resistance of a laminateembodying the present invention are superior to those of thepostformable decorative laminates heretofore in commercial use, in whichthe barrier sheet contains a reaction product of formaldehyde andmelamine which is either incompletely cured, or is modified byincorporation of a plasticizing agent that is intended to minimize thebrittleness of the barrier sheet.

Postformable laminates embodying the present invention, because of theirsuperior postformability, give more satisfactory results in themanufacture of contoured table tops and counter tops than thepostformable decorative laminates heretofore available. Also, structuresmade from the present postformable laminates perform more satisfactorilyin use because of the superior durability of the surface of the presentpostformable laminates. The properties which impart superiorpostformability to the present laminates make it possible to obtainsuperior results in various manufacturing operations, such as hotpunching, in which the present laminates may be used.

The superior postformability of laminates embodying the presentinvention is demonstrated by the following examples:

EXAMPLE l The postformability of laminate A, embodying the presentinvention, was compared with the postformability of a conventionalpostformable laminate heretofore in commercial use. In order to providean accurate comparison, this conventional commercial laminate wasprepared by a procedure which was the same as the procedure used forpreparing Laminate A except that, instead of the unmodified reactionproduct of formaldehyde and melamine in a mole ratio of 1.521, theaqueous solution used to produce the impregnated barrier sheet containeda commercial formaldehydetoluene sulfonamide-melamine reaction productwhich was ofa standard grade conventionally used for the impregnation ofthe barrier sheet in the production of postformable laminates. The moleratio of formaldehyde to melamine in this reaction product was between2:] and 2.5: l.

The test specimens used for testing postformability consisted of piecesof laminate 2 inches by 8 inches in size, including four pieces cut fromLaminate A and four pieces cut from the conventional commercial laminateprepared as hereinbefore described. The method used for testing thepostformability of the four specimens of each laminate was the methoddescribed in the NEMA Standards Publication mentioned above. The bendingjig used for testing postformability by this method had a lower cavityin the shape of a notch. This cavity was provided with interchangeableinserts which determined the radius at the apex of the notch. Fourdifferent radii were used for the four specimens cut from each laminate.The bending jig also included a vertically movable member consisting ofa 90 wedge arranged to fit in the cavity. The edge of the 90 wedge wasslightly rounded. In the testing of each specimen, the specimen wasfirst placed in a standard infra red heater, the potential across theheating elements being volts. In this heater the specimen was locatedthree inches above the heating elements, and was arranged with thebarrier sheet facing downward. As the temperature indicator, a standardindicator wax was applied to the back of each specimen. As soon as thewax melted, indicating that the specimen had reached a temperature of325 F., the specimen was placed in the cavity of the bending jig, withthe barrier sheet still facing downward. The movable wedge portion ofthe jig was then lowered with sufficient force so that the time elapsedbetween the instant when the wedge came in contact with the back of thespecimen and the instant when the barrier sheet came into full contactwith the contoured notch was slightly less than 1 second. The specimenwas then held under pressure in contact with the contoured notch for oneminute in order to allow the specimen to cool.

The results of the postformability tests were as follows:

POSTFORMABILITY OF LAMINATES (15 Minute Cure) Post- Appearance ofsurface Forming Radius at bend (inches) Laminate A Conventional laminateExcellent Excellent 54; Excellent Excellent 5/16 Excellent Cracked 3/16Excellent Cracked These results show that the postformability ofLaminate A, embodying the present invention, was much better than thatof the conventional commercial laminate, in that the barrier sheet ofLaminate A did not crack even when Laminate A was post-formed to aradius as small as threesixteenths inch, whereas the barrier sheet ofthe conventional commercial laminate was cracked even when an attemptwas made to postform the conventional commercial laminate to a radius aslarge as five-sixteenths inch.

According to the NEMA Standards Publication which describes the testprocedures, a postformable laminate is considered to be of a standardcommercial grade if it can be post-formed to a radius at least as smallas three-fourths inch without cracking or dulling of the surface. Thusit is evident that the present invention makes possible the attainmentof a degree of postformability that is vastly superior to the degree ofpostformability which has been considered commercially acceptableheretofore.

The conventional laminate which was prepared and tested as heretoforedescribed was a typical example of the type of postformable laminatewhich has been in commercial use heretofore. In this conventional typeof postformable laminate, the, barrier sheet is impregnated with amelamine-formaldehyde resin containing a plasticizing agent which,because of its degrading ac tion upon the resin, is used in only alimited amount, so as to strike a balance between poor postformabilityand poor durability.

Thus Laminate A, in which the barrier sheet was impregnated with anunplasticized, fully cured formaldehyde-melamine reaction product, wasnot only su perior in postformability to the conventional laminate usedin the tests, but was also substantially superior to the conventionallaminate in water resistance, chemical resistance and heat resistance ofthe surface formed by the barrier sheet.

Although the water resistance of the conventional laminate used in thesetests was substantially less than the water resistance of Laminate A,the barrier sheet of this conventional laminate, containing aplasticized formaldehyde-melamine resin, was found to be fully curedwhen this conventional laminate was subjected to the cure test,described in the NEMA Standards, Publication, which was used in testingthe cure of Laminate A.

EXAMPLE 2 In order to determine the effect of increasing the cure time,another postformable laminate embodying the present invention wasprepared in the same manner as Laminate A, except that the period oftime for which the laminate was held under pressure at a temperaturebetween 260 and 265 F. was 17 minutes instead of 15 minutes. Forpurposes of comparison, another conventional laminate was prepared by aprocedure which was the same as that used in Example I except that theconventional laminate was held at a temperature between 260 and 265 F.for 17 minutes instead of 15 minutes.

Four samples cut from each of these two laminates were then tested forpostformability in the manner described in Example 1, with the followingresults:

POSTFORMABILITY TESTS 17 Minute Cure) Post- Appearance of surfaceForming Radius at bend (inches) Laminate A Conventional laminate 3kExcellent Excellent is Excellent Cracked 5/16 Excellent Cracked 3/16Slightly Cracked Cracked The above results indicate that both types oflaminates, which were fully cured in minutes, were slightly overcured in17 minutes, as indicated by increased brittleness. However, the aboveresults still show that the postformability of the laminate embodyingthe present invention was greatly superior to the postformability of theconventional laminate, in that the laminate of the present invention wasslightly cracked when post-formed at a radius as small asthreesixteenths inch, while the conventional laminate was cracked whenpost-formed at a radius as large as fiveeighths inch.

EXAMPLE 3 In order to determine the effect of decreasing the cure time,an additional laminate embodying the present invention was prepared by aprocedure which was the same as the procedure used in preparing LaminateA, except that the time for which the laminate was held under pressureat a temperature between 260 and 265 F. was 13 minutes instead of 15minutes. For purposes of comparison, a laminate of the conventional typewas prepared by a procedure which was the same as the procedure used inpreparing the conventional laminate in Example 1 except that thelaminate was held under pressure at a temperature between 260 and 265 F.for 13 minutes instead of 15 minutes.

Four samples cut from each of these two laminates were tested forpostformability by the test procedure used in Example 1, with thefollowing results:

POSTFORMABILITY TESTS (13 Minute Cure) Post- Appearance of surfaceForming Radius at bend (inches) Laminate A Conventional laminate 54Excellent Dull 9i: Excellent Dull 5/16 Excellent Dull 3/16 ExcellentDull The foregoing results show that Laminate A, embodying the presentinvention, had excellent postformability even when the cure time wasonly 13 minutes. The conventional laminate, when cured for only 13minutes, was incompletely cured and commercially unacceptable under theNEMA Standards, as shown by the fact that the surface of the barriersheet in the area of the bend became dull upon reheating during thepostforming operation.

The laminates prepared in accordance with this example, which were curedfor only 13 minutes, were also subjected to the cure test as used fortesting the cure of Laminate A, in accordance with the proceduredescribed in the NEMA Standards Publication. In this cure test, nodeterioration of the surface of the laminate embodying the inventionwhich had been cured for only 13 minutes was observed, indicating thatthe formaldehyde-melamine resin in the barrier sheet of that laminatehadbeen fully cured. On the other hand, when the same cure test was appliedto the conventional laminate which had been cured for only 13 minutes,the barrier sheet of that laminate became dull and splotchy, indicatingthat the conventional laminate was definitely undercured when a curetime of only 13 minutes was used.

These results also show that the: relative brittleness of theconventional laminate tested in Example 1 was not due to over-curing,because the 13 minute cure employed in Example 3 was insufficient tocomplete the cure of the conventional laminate, and the 15 minute cureemployed in Example 1 was just sufficient to cure the conventionallaminate.

These postformability tests on the specimens of the conventional type ofpostformable laminate show that the limited amount of plasticizing agentwhich can be used in the conventional type of laminate imparts onlymarginal postformability. The 15 minute cure time used in Example 1 wasjust sufficient to cure the conventional type of laminate, but thepostformability tests in Example 1 showed that the curing of theconventional type of laminate was accompanied by simultaneousdevelopment of brittleness which caused the conventional laminate tocrack when an attempt was made to postform it to a radius less thanfive-eighths inch. This brittleness of the conventional type ofpostformable laminate is due to the fact that only a limited amount ofthe plasticizing agent can be incorporated in the resin with which thebarrier sheet is impregnated without causing prohibitive degradation ofthe durability of the conventional type of laminate.

In the production of Laminate A, the acidity of the unplasticizedformaldehyde-melamine resin with which the barrier sheet was impregnatedwas high enough to compensate for its low formaldehyde content so as toforce the cure of the resin to completion even when the cure time wasonly 13 minutes, as shown by the results in Example 3. Theformaldehyde-melamine resin with which the barrier sheet of Laminate Awas impregnated was remarkably non-brittle and provided excellentpostformability whether cured for 13 minutes or for 15 minutes. Evenwhen cured for 17 minutes, the unplasticized formaldehyde-melamine resinin the barrier sheet of Laminate A, although slightly overcured, wasstill much less brittle than the plasticized formaldehyde-melamine resinin the barrier sheet of the conventional type of laminate tested inExample 1, which had been cured just long enough to complete the cure ofthe resin.

Thus the results obtained in manufacturing the postformable laminates ofthe present invention are much more reliable and reproducible than theresults obtained in manufacturing the conventional type of postformablelaminate heretofore commercially used, because the curing of thelaminates of the present invention is a much less critical operation.The results obtained in these examples show thatLaminate A, embodyingthe present invention, has excellent postformability, whether cured for13, 15 or 17 minutes. In contrast, the postformability of theconventional type of laminate used in Example 1 was very marginal eventhough that laminate had been cured just long enough to complete thecure of the plasticized formaldehydemelamine resin in the barrier sheet,using a cure time of 15 minutes. When the cure time of the conventionaltype of postformable laminate was increased slightly to 17 minutes, asin Example 2, the postformability of the conventional type of laminatewas seriously impaired.

I claim:

1. A method of producing a postformable laminate having a paper barriersheet of wood fiber impregnated with a fully cured unplasticizedmelamine-formaldehyde resin, comprising the step of consolidating underheat and pressure in a dry state a paper barrier sheet and apostformable substrate in direct contact with the barrier sheet,characterized in that the paper barrier sheet consists essentially ofalpha cellulose impregnated with an unplasticized reaction product offormaldehyde and melamine in a mole ratio from 1.1 :1 to 1.8:1, havingan acidity high enough to compensate for its low formaldehyde content soas to cause it to become fully cured during the heatandpressure-consolidation, the resulting laminate being postformable to anarc of a circle having a radius at least as small as three-fourths inchwithout cracking or dulling of the surface.

2. A method according to claim 1 wherein the paper barrier sheetconsists essentially of alpha cellulose impregnated with anunplasticized reaction product of formaldehyde and melamine in a moleratio from 1.211 to 1.7:1.

3. A heatand pressure-consolidated postformable laminate comprising apaper barrier sheet of wood fiber which is impregnated with a fullycured unplasticized melamine-formaldehyde resin and is directly bondedto a postformable substrate, characterized in that the paper barriersheet consists essentially of alpha cellulose impregnated with anunplasticized reaction product of formaldehyde and melamine in a moleratio from 1.1:1 to 1.811 which has been fully cured during the heatandpressure-consolidation of the laminate in a dry state at an acidity highenough to compensate for its low formaldehyde content and to permitcomplete cure, the laminate being postformable to an arc of a circlehaving a radius at least as small as three-fourths inch without crackingor dulling of the surface.

4. A postformable laminate according to claim 3 wherein the paperbarrier sheet consists essentially of alpha cellulose impregnated withan unplasticized reaction product of formaldehyde and melamine in a moleratio from 1.221 to 1.7:1.

5. A postformable laminate according to claim 3 comprising an overlaysheet of paper which is bonded to the barrier sheet and consistsessentially of alpha cellulose impregnated with a fully curedunplasticized reaction product of formaldehyde and melamine in a moleratio from 1.1:1 to 1.821.

2. A method according to claim 1 wherein the paper barrier sheetconsists essentially of alpha cellulose impregnated with anunplasticized reaction product of formaldehyde and melamine in a moleratio from 1.2:1 to 1.7:1.
 3. A heat- and pressure-consolidatedpostformable laminate comprising a paper barrier sheet of wood fiberwhich is impregnated with a fully cured unplasticizedmelamine-formaldehyde resin and is directly bonded to a postformablesubstrate, characterized in that the paper barrier sheet consistsessentially of alpha cellulose impregnated with an unplasticizedreaction product of formaldehyde and melamine in a mole ratio from 1.1:1to 1.8:1 which has been fully cured during the heat-andpressure-consolidation of the laminate in a dry state at an acidity highenough to compensate for its low formaldehyde content and to permitcomplete cure, the laminate being postformable to an arc of a circlehaving a radius at least as small as three-fourths inch without crackingor dulling of the surface.
 4. A postformable laminate according to claim3 wherein the paper barrier sheet consists essentially of alphacellulose impregnated with an unplasticized reaction product offormaldehyde and melamine in a mole ratio from 1.2:1 to 1.7:1.
 5. Apostformable laminate according to claim 3 comprising an overlay sheetof paper which is bonded to the barrier sheet and consists essentiallyof alpha cellulose impregnated with a fully cured unplasticized reactionproduct of formaldehyde and melamine in a mole ratio from 1.1:1 to1.8:1.